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      流逝的是岁月，不变的是情怀.
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      坚持学习，是为了成就更好的自己. <br></div> <div>公众号[中关村程序员]</div></div></div> <!----></div> <div class="theme-default-content content__default"><h2 id="_1-1-今日目标"><a href="#_1-1-今日目标" class="header-anchor">#</a> 1.1 今日目标</h2> <ol><li>理解实体之间的关系</li> <li>理解绘制E-R图</li> <li>理解三范式</li> <li>理解范式和性能的关系</li> <li>能够查询表中的数据</li> <li>理解聚合函数</li> <li>理解模糊查询</li> <li>理解分组查询</li></ol> <h2 id="_1-2-数据库基本概念"><a href="#_1-2-数据库基本概念" class="header-anchor">#</a> 1.2  数据库基本概念</h2> <p>1、关系：两个表的公共字段</p> <p>2、行：也称记录，也称实体</p> <p>3、列：也称字段，也称属性</p> <div class="language- extra-class"><pre class="language-text"><code>就表结构而言，表分为行和列；
就表数据而言，分为记录和字段；
就面向对象而言，一个记录就是一个实体，一个字段就是一个属性。
</code></pre></div><p>4、数据冗余：相同的数据存储在不同的地方</p> <div class="language- extra-class"><pre class="language-text"><code>脚下留心：
1、冗余只能减少，不能杜绝。
2、减少冗余的方法是分表
3、为减少数据查找的麻烦，允许数据有一定的冗余
</code></pre></div><p>5、数据完整性：正确性+准确性=数据完整性</p> <div class="language- extra-class"><pre class="language-text"><code>正确性：数据类型正确
准确性：数据范围要准确
</code></pre></div><p>思考：学生的年龄是整型，输入1000岁，正确性和准确性如何？</p> <p>答：正确的，但不准确</p> <p>思考：年龄是整形的，收入了字符串，正确性和准确性如何？</p> <p>答：不正确</p> <h2 id="_1-3-实体和实体之间的关系"><a href="#_1-3-实体和实体之间的关系" class="header-anchor">#</a> 1.3  实体和实体之间的关系</h2> <p>1、一对一</p> <p>2、一对多 （多对一）</p> <p>3、多对多 <img src="http://static.zhequtao.com/study/mysql/day3/1536975724695.png" alt="1536975724695"></p> <h4 id="_1-3-1-一对多-1：n"><a href="#_1-3-1-一对多-1：n" class="header-anchor">#</a> 1.3.1 一对多 1：N</h4> <p>1、主表中的一条记录对应从表中的多条记录。</p> <p>2、一对多和多对一是一样的</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536976096582.png" alt="1536976096582"></p> <hr> <p>如何实现一对多？</p> <p>答：主键和非主键建关系</p> <p>问题：说出几个一对多的关系？</p> <p>答：班级表和学生表、      班主表和学生表</p> <h4 id="_1-3-2-一对一（1-1）"><a href="#_1-3-2-一对一（1-1）" class="header-anchor">#</a> 1.3.2   一对一（1:1）</h4> <p>1、主表中的一条记录对应从表中的一条记录</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536976513236.png" alt="1536976513236"></p> <p>如何实现一对一？</p> <p>主键和主键建关系就能实现一对一。</p> <div class="language- extra-class"><pre class="language-text"><code>思考：一对一两个表完全可以用一个表实现，为什么还要分成两个表？

答：在字段数量很多情况下，数据量也就很大，每次查询都需要检索大量数据，这样效率低下。我们可以将所有字段分成两个部分，“常用字段”和“不常用字段”，这样对大部分查询者来说效率提高了。【表的垂直分割】
</code></pre></div><h4 id="_1-3-3-多对多（n：m）"><a href="#_1-3-3-多对多（n：m）" class="header-anchor">#</a> 1.3.3   多对多（N：M）</h4> <p>主表中的一条记录对应从表中的多条记录，从表中的一条记录对应主表中的多条记录</p> <p>班级和讲师的关系</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536977577667.png" alt="1536977577667"></p> <p>如何实现多对多？</p> <p>答：建立第三张表来保存关系。</p> <p>问题：说出几个多对多的关系？</p> <p>1、科目表和学生表的关系		2、商品表和订单表         3、游戏目录表和玩家表</p> <h2 id="_1-4-数据库设计的步骤"><a href="#_1-4-数据库设计的步骤" class="header-anchor">#</a> 1.4 数据库设计的步骤</h2> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536977906540.png" alt="1536977906540"></p> <h4 id="_1-4-1-数据库设计具体步骤"><a href="#_1-4-1-数据库设计具体步骤" class="header-anchor">#</a> 1.4.1   数据库设计具体步骤</h4> <p>1、     收集信息：与该系统有关人员进行交流、坐谈，充分理解数据库需要完成的任务</p> <p>2、     标识对象（实体－Entity）标识数据库要管理的关键对象或实体</p> <p>3、     标识每个实体的属性（Attribute）</p> <p>4、     标识对象之间的关系（Relationship）</p> <p>5、     将模型转换成数据库</p> <p>6、     规范化</p> <h4 id="_1-4-2-绘制e-r图"><a href="#_1-4-2-绘制e-r图" class="header-anchor">#</a> 1.4.2   绘制E-R图</h4> <p>E-R（Entity－Relationship）实体关系图</p> <p>E-R图的语法</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536978470142.png" alt="1536978470142"></p> <p>绘制E-R图</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536978565499.png" alt="1536978565499"></p> <h4 id="_1-4-3-将e-r图转成表"><a href="#_1-4-3-将e-r图转成表" class="header-anchor">#</a> 1.4.3   将E-R图转成表</h4> <p>1、     实体转成表，属性转成字段</p> <p>2、     如果没有合适的字段做主键，给表添加一个自动增长列做主键。</p> <h4 id="_1-4-4-例题"><a href="#_1-4-4-例题" class="header-anchor">#</a> 1.4.4  例题</h4> <p>1、项目需求</p> <div class="language- extra-class"><pre class="language-text"><code>BBS论坛的基本功能：
用户注册和登录，后台数据库需要存放用户的注册信息和在线状态信息；
用户发贴，后台数据库需要存放贴子相关信息，如贴子内容、标题等；
用户可以对发帖进行回复；
论坛版块管理：后台数据库需要存放各个版块信息，如版主、版块名称、贴子数等；
</code></pre></div><p>2、标识对象</p> <div class="language- extra-class"><pre><code>参与的对象有：用户、发的帖子、跟帖、板块
</code></pre></div><p>3、标识对象的属性</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536978271446.png" alt="1536978271446"></p> <p>4、建立关系，绘制E-R图</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536978734804.png" alt="1536978734804"></p> <p>5、将E-R图转出表结构</p> <h2 id="_1-5-数据规范化"><a href="#_1-5-数据规范化" class="header-anchor">#</a> 1.5 数据规范化</h2> <p>Codd博士定义了6个范式来规范化数据库，范式由小到大来约束，范式越高冗余越小，但表的个数也越多。实验证明，三范式是性价比最高的。</p> <h4 id="_1-5-1-第一范式：确保每列原子性"><a href="#_1-5-1-第一范式：确保每列原子性" class="header-anchor">#</a> 1.5.1   第一范式：确保每列原子性</h4> <p>第一范式确保每个字段不可再分</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536979522448.png" alt="1536979522448"></p> <p>思考：如下表设计是否合理？</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536979672041.png" alt="1536979672041"></p> <p>不合理。不满足第一范式，上课时间可以再分</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536979718323.png" alt="1536979718323"></p> <p>思考：地址包含省、市、县、地区是否需要拆分？</p> <p>答：如果仅仅起地址的作用，不需要统计，可以不拆分；如果有按地区统计的功能需要拆分。</p> <p>在实际项目中，建议拆分。</p> <h4 id="_1-5-2-第二范式：非键字段必须依赖于键字段"><a href="#_1-5-2-第二范式：非键字段必须依赖于键字段" class="header-anchor">#</a> 1.5.2   第二范式：非键字段必须依赖于键字段</h4> <p>一个表只能描述一件事</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536980482113.png" alt="1536980482113"></p> <p>思考：如下表设计是否合理？</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536980574792.png" alt="1536980574792"></p> <h4 id="_1-5-3-第三范式：消除传递依赖"><a href="#_1-5-3-第三范式：消除传递依赖" class="header-anchor">#</a> 1.5.3   第三范式：消除传递依赖</h4> <p>在所有的非键字段中，不能有传递依赖</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536981861980.png" alt="1536981861980"></p> <p>下列设计是否满足第三范式？</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536982061259.png" alt="1536982061259"></p> <p>不满足，因为语文和数学确定了，总分就确定了。</p> <div class="language- extra-class"><pre class="language-text"><code>多学一招：上面的设计不满足第三范式，但是高考分数表就是这样设计的，为什么？

答：高考分数峰值访问量非常大，这时候就是性能更重要。当性能和规范化冲突的时候，我们首选性能。这就是“反三范式”。
</code></pre></div><h4 id="_1-5-4-数据库设计的例题"><a href="#_1-5-4-数据库设计的例题" class="header-anchor">#</a> 1.5.4  数据库设计的例题</h4> <p>1、需求</p> <div class="language- extra-class"><pre class="language-text"><code>公司承担多个工程项目，每一项工程有：工程号、工程名称、施工人员等
公司有多名职工，每一名职工有：职工号、姓名、性别、职务（工程师、技术员）等
公司按照工时和小时工资率支付工资，小时工资率由职工的职务决定（例如，技术员的小时工资率与工程师不同）
</code></pre></div><p>2、工资表</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536982976983.png" alt="1536982976983"></p> <p>3、将工资表转成数据库表</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536983011535.png" alt="1536983011535"></p> <p>4、这个表存在的问题</p> <div class="language- extra-class"><pre><code>A：新人入职需要虚拟一个项目

B：职务更改，小时工资率可能会忘记更改，造成数据不完整

C：有人离职，删除记录后，工程也没有了
</code></pre></div><p>5、规范化表</p> <div class="language- extra-class"><pre><code>第一步：这个表满足第一范式

第二步：这个表不是描述了一件事情
</code></pre></div><p><img src="http://static.zhequtao.com/study/mysql/day3/1536983332419.png" alt="1536983332419"></p> <div class="language- extra-class"><pre><code>第三步：是否满足第三范式
</code></pre></div><p><img src="http://static.zhequtao.com/study/mysql/day3/1536983422611.png" alt="1536983422611"></p> <p>更改如下：</p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536983406042.png" alt="1536983406042"></p> <h2 id="_1-6-查询语句"><a href="#_1-6-查询语句" class="header-anchor">#</a> 1.6 查询语句</h2> <div class="language-sql extra-class"><pre class="language-sql"><code>语法：<span class="token keyword">select</span> <span class="token punctuation">[</span>选项<span class="token punctuation">]</span> 列名 <span class="token punctuation">[</span><span class="token keyword">from</span> 表名<span class="token punctuation">]</span> <span class="token punctuation">[</span><span class="token keyword">where</span> 条件<span class="token punctuation">]</span>  <span class="token punctuation">[</span><span class="token keyword">group</span> <span class="token keyword">by</span> 分组<span class="token punctuation">]</span> <span class="token punctuation">[</span><span class="token keyword">order</span> <span class="token keyword">by</span> 排序<span class="token punctuation">]</span><span class="token punctuation">[</span><span class="token keyword">having</span> 条件<span class="token punctuation">]</span> <span class="token punctuation">[</span><span class="token keyword">limit</span> 限制<span class="token punctuation">]</span>
</code></pre></div><h4 id="_1-6-1-字段表达式"><a href="#_1-6-1-字段表达式" class="header-anchor">#</a> 1.6.1   字段表达式</h4> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token string">'锄禾日当午'</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token operator">|</span> 锄禾日当午          <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token operator">|</span> 锄禾日当午          <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token number">10</span><span class="token operator">*</span><span class="token number">10</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">-------+</span>
<span class="token operator">|</span> <span class="token number">10</span><span class="token operator">*</span><span class="token number">10</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">-------+</span>
<span class="token operator">|</span>   <span class="token number">100</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">-------+</span>
</code></pre></div><p>通过as给字段取别名</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span>  <span class="token keyword">select</span> <span class="token string">'锄禾日当午'</span> <span class="token keyword">as</span> content<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token operator">|</span> content    <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token operator">|</span> 锄禾日当午          <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token number">1</span> <span class="token keyword">row</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token number">10</span><span class="token operator">*</span><span class="token number">10</span> <span class="token keyword">as</span> result<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token operator">|</span> result <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token operator">|</span>    <span class="token number">100</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token number">1</span> <span class="token keyword">row</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><p>多学一招：as可以省略</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token number">10</span><span class="token operator">*</span><span class="token number">10</span>  result<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token operator">|</span> result <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token operator">|</span>    <span class="token number">100</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token number">1</span> <span class="token keyword">row</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><h4 id="_1-6-2-from子句"><a href="#_1-6-2-from子句" class="header-anchor">#</a> 1.6.2   from子句</h4> <p>from：来自，from后面跟的是数据源。数据源可以有多个。返回笛卡尔积。</p> <p>插入测试表</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">create</span> <span class="token keyword">table</span> t1<span class="token punctuation">(</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> id <span class="token keyword">int</span><span class="token punctuation">,</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> name <span class="token keyword">varchar</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
Query OK<span class="token punctuation">,</span> <span class="token number">0</span> <span class="token keyword">rows</span> affected <span class="token punctuation">(</span><span class="token number">0.05</span> sec<span class="token punctuation">)</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">create</span> <span class="token keyword">table</span> t2<span class="token punctuation">(</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> field1 <span class="token keyword">varchar</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> field2 <span class="token keyword">varchar</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
Query OK<span class="token punctuation">,</span> <span class="token number">0</span> <span class="token keyword">rows</span> affected <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">insert</span> <span class="token keyword">into</span> t1 <span class="token keyword">values</span> <span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token string">'tom'</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token string">'berry'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Query OK<span class="token punctuation">,</span> <span class="token number">2</span> <span class="token keyword">rows</span> affected <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
Records: <span class="token number">2</span>  Duplicates: <span class="token number">0</span>  <span class="token keyword">Warnings</span>: <span class="token number">0</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">insert</span> <span class="token keyword">into</span> t2 <span class="token keyword">values</span> <span class="token punctuation">(</span><span class="token string">'333'</span><span class="token punctuation">,</span><span class="token string">'333'</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token punctuation">(</span><span class="token string">'444'</span><span class="token punctuation">,</span><span class="token string">'444'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Query OK<span class="token punctuation">,</span> <span class="token number">2</span> <span class="token keyword">rows</span> affected <span class="token punctuation">(</span><span class="token number">0.02</span> sec<span class="token punctuation">)</span>
Records: <span class="token number">2</span>  Duplicates: <span class="token number">0</span>  <span class="token keyword">Warnings</span>: <span class="token number">0</span>
</code></pre></div><p>测试多个数据源</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> t1<span class="token punctuation">,</span>t2<span class="token punctuation">;</span>              <span class="token comment"># 返回笛卡尔积</span>
<span class="token operator">+</span><span class="token comment">------+-------+--------+--------+</span>
<span class="token operator">|</span> id   <span class="token operator">|</span> name  <span class="token operator">|</span> field1 <span class="token operator">|</span> field2 <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------+-------+--------+--------+</span>
<span class="token operator">|</span>    <span class="token number">1</span> <span class="token operator">|</span> tom   <span class="token operator">|</span> <span class="token number">333</span>    <span class="token operator">|</span> <span class="token number">333</span>    <span class="token operator">|</span>
<span class="token operator">|</span>    <span class="token number">2</span> <span class="token operator">|</span> berry <span class="token operator">|</span> <span class="token number">333</span>    <span class="token operator">|</span> <span class="token number">333</span>    <span class="token operator">|</span>
<span class="token operator">|</span>    <span class="token number">1</span> <span class="token operator">|</span> tom   <span class="token operator">|</span> <span class="token number">444</span>    <span class="token operator">|</span> <span class="token number">444</span>    <span class="token operator">|</span>
<span class="token operator">|</span>    <span class="token number">2</span> <span class="token operator">|</span> berry <span class="token operator">|</span> <span class="token number">444</span>    <span class="token operator">|</span> <span class="token number">444</span>    <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------+-------+--------+--------+</span>
<span class="token number">4</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><h4 id="_1-6-3-dual表"><a href="#_1-6-3-dual表" class="header-anchor">#</a> 1.6.3   dual表</h4> <p>dual表是一个伪表。在有些特定情况下，没有具体的表的参与，但是为了保证select语句的完整又必须要一个表名，这时候就使用伪表。</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token number">10</span><span class="token operator">*</span><span class="token number">10</span> <span class="token keyword">as</span> result <span class="token keyword">from</span> dual<span class="token punctuation">;</span>  <span class="token comment">#dual表是用来保证select语句的完整性。</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token operator">|</span> result <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
<span class="token operator">|</span>    <span class="token number">100</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+</span>
</code></pre></div><h4 id="_1-6-4-where子句"><a href="#_1-6-4-where子句" class="header-anchor">#</a> 1.6.4   where子句</h4> <p>where后面跟的是条件，在数据源中进行筛选。返回条件为真记录</p> <p>MySQL支持的运算符</p> <ol><li><code>&gt;</code>	大于</li> <li><code>&lt;</code>小于</li> <li><code>&gt;=</code></li> <li><code>&lt;=</code></li> <li><code>=</code></li> <li><code>!=</code></li> <li>and    与</li> <li>or      或</li> <li>not   非</li></ol> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stusex<span class="token operator">=</span><span class="token string">'男'</span><span class="token punctuation">;</span>		<span class="token comment"># 查找性别是男的记录</span>
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuage<span class="token operator">&gt;=</span><span class="token number">20</span><span class="token punctuation">;</span>		<span class="token comment"># 查找年龄不低于20的记录</span>
</code></pre></div><p>思考：如下代码输出什么</p> <div class="language-sql extra-class"><pre class="language-sql"><code><span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> <span class="token number">1</span>      <span class="token comment"># 返回所有数据库</span>
<span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> <span class="token number">0</span>		<span class="token comment">#返回空记录</span>
</code></pre></div><p>思考：如何查找北京和上海的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuaddress<span class="token operator">=</span><span class="token string">'上海'</span> <span class="token operator">or</span> stuaddress<span class="token operator">=</span><span class="token string">'北京'</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才       <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31</span> <span class="token operator">|</span>       <span class="token number">3</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">76</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25303 <span class="token operator">|</span> 李斯文       <span class="token operator">|</span> 女      <span class="token operator">|</span>     <span class="token number">22</span> <span class="token operator">|</span>       <span class="token number">2</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">55</span> <span class="token operator">|</span>   <span class="token number">82</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25320 <span class="token operator">|</span> Tom     <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">24</span> <span class="token operator">|</span>       <span class="token number">8</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">65</span> <span class="token operator">|</span>   <span class="token number">67</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
</code></pre></div><h4 id="_1-6-5-in-not-in"><a href="#_1-6-5-in-not-in" class="header-anchor">#</a> 1.6.5   in | not in</h4> <p>上面的查询上海和北京的学生的SQL可以通过in语句来实现</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuaddress <span class="token operator">in</span> <span class="token punctuation">(</span><span class="token string">'北京'</span><span class="token punctuation">,</span><span class="token string">'上海'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>练习：</p> <p>1、查找学号是s25301,s25302,s25303的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuno <span class="token operator">in</span> <span class="token punctuation">(</span><span class="token string">'s25301'</span><span class="token punctuation">,</span><span class="token string">'s25302'</span><span class="token punctuation">,</span><span class="token string">'s25303'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>2、查找年龄是18,19,20的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuage <span class="token operator">in</span><span class="token punctuation">(</span><span class="token number">18</span><span class="token punctuation">,</span><span class="token number">19</span><span class="token punctuation">,</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>3、查找不是北京和上海的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuaddress <span class="token operator">not</span> <span class="token operator">in</span> <span class="token punctuation">(</span><span class="token string">'北京'</span><span class="token punctuation">,</span><span class="token string">'上海'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><h4 id="_1-6-6-between…and-not-between…and"><a href="#_1-6-6-between…and-not-between…and" class="header-anchor">#</a> 1.6.6   between…and|not between…and</h4> <p>查找某个范围的记录</p> <p>1、查找年龄在18~20之间的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuage<span class="token operator">&gt;=</span><span class="token number">18</span> <span class="token operator">and</span> stuage<span class="token operator">&lt;=</span><span class="token number">20</span><span class="token punctuation">;</span>   <span class="token comment"># 方法一</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuage <span class="token operator">between</span> <span class="token number">18</span> <span class="token operator">and</span> <span class="token number">20</span><span class="token punctuation">;</span>   <span class="token comment"># 方法二</span>
</code></pre></div><p>2、查找年龄不在18~20之间的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuage<span class="token operator">&lt;</span><span class="token number">18</span> <span class="token operator">or</span> stuage<span class="token operator">&gt;</span><span class="token number">20</span><span class="token punctuation">;</span>		<span class="token comment">#方法一</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> <span class="token operator">not</span> <span class="token punctuation">(</span>stuage<span class="token operator">&gt;=</span><span class="token number">18</span> <span class="token operator">and</span> stuage<span class="token operator">&lt;=</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuage <span class="token operator">not</span> <span class="token operator">between</span> <span class="token number">18</span> <span class="token operator">and</span> <span class="token number">20</span><span class="token punctuation">;</span>
</code></pre></div><h4 id="_1-6-7-is-null-is-not-null"><a href="#_1-6-7-is-null-is-not-null" class="header-anchor">#</a> 1.6.7   is null | is not null</h4> <p>脚下留心：查询一个为空的字段不能用等于，必须用is null</p> <p>查找缺考的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> ch <span class="token operator">is</span> <span class="token boolean">null</span> <span class="token operator">or</span> math <span class="token operator">is</span> <span class="token boolean">null</span><span class="token punctuation">;</span> <span class="token comment"># 查找缺考的人</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName  <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽         <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25304 <span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">28</span> <span class="token operator">|</span>       <span class="token number">4</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>   <span class="token number">74</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
</code></pre></div><p>查找参加考试的学生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> ch <span class="token operator">is</span> <span class="token operator">not</span> <span class="token boolean">null</span> <span class="token operator">and</span> math <span class="token operator">is</span> <span class="token operator">not</span> <span class="token boolean">null</span><span class="token punctuation">;</span>
</code></pre></div><h4 id="_1-6-8-聚合函数"><a href="#_1-6-8-聚合函数" class="header-anchor">#</a> 1.6.8   聚合函数</h4> <ol><li><p>sum()  	   求和</p></li> <li><p>avg()          求平均值</p></li> <li><p>max()        求最大值</p></li> <li><p>min()          求最小值</p></li> <li><p>count()      求记录数</p></li></ol> <div class="language-sql extra-class"><pre class="language-sql"><code><span class="token comment">#求语文总分、语文平均分、语文最高分、语文最低分、总人数</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token function">sum</span><span class="token punctuation">(</span>ch<span class="token punctuation">)</span> <span class="token string">'语文总分'</span><span class="token punctuation">,</span><span class="token function">avg</span><span class="token punctuation">(</span>ch<span class="token punctuation">)</span> <span class="token string">'语文平均分'</span><span class="token punctuation">,</span> <span class="token function">max</span><span class="token punctuation">(</span>ch<span class="token punctuation">)</span> <span class="token string">'语文最高分'</span><span class="token punctuation">,</span><span class="token function">min</span><span class="token punctuation">(</span>ch<span class="token punctuation">)</span> <span class="token string">'语文最低分'</span><span class="token punctuation">,</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token operator">*</span><span class="token punctuation">)</span> <span class="token string">'总人数'</span> <span class="token keyword">from</span> stu<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">----------+------------+------------+------------+--------+</span>
<span class="token operator">|</span> 语文总分        <span class="token operator">|</span> 语文平均分          <span class="token operator">|</span> 语文最高分          <span class="token operator">|</span> 语文最低分          <span class="token operator">|</span> 总人数       <span class="token operator">|</span>

<span class="token operator">+</span><span class="token comment">----------+------------+------------+------------+--------+</span>
<span class="token operator">|</span>      <span class="token number">597</span> <span class="token operator">|</span>    <span class="token number">74.6250</span> <span class="token operator">|</span>         <span class="token number">88</span> <span class="token operator">|</span>         <span class="token number">55</span> <span class="token operator">|</span>      <span class="token number">9</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">----------+------------+------------+------------+--------+</span>
<span class="token number">1</span> <span class="token keyword">row</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><h4 id="_1-6-9-通配符"><a href="#_1-6-9-通配符" class="header-anchor">#</a> 1.6.9   通配符</h4> <ol><li><p>_  [下划线]   表示任意一个字符</p></li> <li><p>%  表示任意字符</p></li></ol> <p>练习</p> <p>1、满足“T_m”的有（A、C）</p> <p>A：Tom         B：Toom       C：Tam         D：Tm     E：Tmo</p> <p>2、满足“T_m_”的有（B、C  ）</p> <p>A:Tmom   B:Tmmm  C:T1m2    D:Tmm     E:Tm</p> <p>3、满足“张%”的是（A、B、C、D）</p> <p>A:张三     B：张三丰     C：张牙舞爪      D：张      E：小张</p> <p>4、满足“%诺基亚%”的是（A、B、C、D）</p> <p>A：诺基亚2100   B：2100诺基亚   C：把我的诺基亚拿过来   D：诺基亚</p> <h4 id="_16-10-模糊查询（like）"><a href="#_16-10-模糊查询（like）" class="header-anchor">#</a> 16.10 模糊查询（like）</h4> <div class="language-sql extra-class"><pre class="language-sql"><code><span class="token comment"># 查找姓张的同学</span>
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuname <span class="token operator">like</span> <span class="token string">'张%'</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token number">1</span> <span class="token keyword">row</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
<span class="token comment">#例题</span>
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuname <span class="token operator">like</span> <span class="token string">'T_m'</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25320 <span class="token operator">|</span> Tom     <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">24</span> <span class="token operator">|</span>       <span class="token number">8</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">65</span> <span class="token operator">|</span>   <span class="token number">67</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token number">1</span> <span class="token keyword">row</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><h4 id="_1-6-11-order-by排序"><a href="#_1-6-11-order-by排序" class="header-anchor">#</a> 1.6.11 order by排序</h4> <p>asc：升序【默认】</p> <p>desc：降序</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> ch <span class="token keyword">desc</span><span class="token punctuation">;</span>		<span class="token comment"># 语文成绩降序排列</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> math <span class="token keyword">asc</span><span class="token punctuation">;</span>     <span class="token comment"># 数学成绩升序排列</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> math<span class="token punctuation">;</span>       <span class="token comment"># 默认升序排列</span>
</code></pre></div><p>多列排序</p> <div class="language-sql extra-class"><pre class="language-sql"><code><span class="token comment">#年龄升序,成绩降序</span>
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span><span class="token punctuation">,</span><span class="token punctuation">(</span>ch<span class="token operator">+</span>math<span class="token punctuation">)</span> <span class="token keyword">as</span> <span class="token string">'总分'</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> stuage <span class="token keyword">asc</span><span class="token punctuation">,</span><span class="token punctuation">(</span>ch<span class="token operator">+</span>math<span class="token punctuation">)</span> <span class="token keyword">desc</span><span class="token punctuation">;</span>
</code></pre></div><p>思考如下代码表示什么含义</p> <div class="language-sql extra-class"><pre class="language-sql"><code><span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> stuage <span class="token keyword">desc</span><span class="token punctuation">,</span>ch <span class="token keyword">desc</span><span class="token punctuation">;</span>     <span class="token comment">#年龄降序，语文降序</span>
<span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> stuage <span class="token keyword">desc</span><span class="token punctuation">,</span>ch <span class="token keyword">asc</span><span class="token punctuation">;</span>		<span class="token comment">#年龄降序，语文升序</span>
<span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> stuage<span class="token punctuation">,</span>ch <span class="token keyword">desc</span><span class="token punctuation">;</span>          <span class="token comment">#年龄升序、语文降序</span>
<span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> stuage<span class="token punctuation">,</span>ch<span class="token punctuation">;</span> 				<span class="token comment">#年龄升序、语文升序</span>
</code></pre></div><h4 id="_1-6-12-group-by-【分组查询】"><a href="#_1-6-12-group-by-【分组查询】" class="header-anchor">#</a> 1.6.12 group by 【分组查询】</h4> <p>将查询的结果分组，分组查询目的在于统计数据。</p> <div class="language-sql extra-class"><pre class="language-sql"><code><span class="token comment"># 按性别分组，显示每组的平均年龄</span>
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token function">avg</span><span class="token punctuation">(</span>stuage<span class="token punctuation">)</span> <span class="token keyword">as</span> <span class="token string">'年龄'</span><span class="token punctuation">,</span>stusex <span class="token keyword">from</span> stu <span class="token keyword">group</span> <span class="token keyword">by</span> stusex<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">---------+--------+</span>
<span class="token operator">|</span> 年龄        <span class="token operator">|</span> stusex <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+--------+</span>
<span class="token operator">|</span> <span class="token number">22.7500</span> <span class="token operator">|</span> 女      <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">25.4000</span> <span class="token operator">|</span> 男       <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+--------+</span>
<span class="token number">2</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
<span class="token comment"># 按地区分组，每个地区的平均年龄</span>
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token function">avg</span><span class="token punctuation">(</span>stuage<span class="token punctuation">)</span> <span class="token keyword">as</span> <span class="token string">'年龄'</span><span class="token punctuation">,</span>stuaddress <span class="token keyword">from</span> stu <span class="token keyword">group</span> <span class="token keyword">by</span> stuaddress<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">---------+------------+</span>
<span class="token operator">|</span> 年龄        <span class="token operator">|</span> stuaddress <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+------------+</span>
<span class="token operator">|</span> <span class="token number">31.0000</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">21.3333</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">27.0000</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">23.0000</span> <span class="token operator">|</span> 河北          <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">23.0000</span> <span class="token operator">|</span> 河南           <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+------------+</span>
<span class="token number">5</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><div class="language- extra-class"><pre class="language-text"><code>脚下留心：
1、如果是分组查询，查询字段必须是分组字段和聚合函数。
2、查询字段是普通字段，只取第一个值
</code></pre></div><p><img src="http://static.zhequtao.com/study/mysql/day3/1536999666442.png" alt="1536999666442"></p> <p>通过group_concat()函数将同一组的值连接起来显示</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> group_concat<span class="token punctuation">(</span>stuname<span class="token punctuation">)</span><span class="token punctuation">,</span>stusex <span class="token keyword">from</span> stu <span class="token keyword">group</span> <span class="token keyword">by</span> stusex<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">-------------------------------------+--------+</span>
<span class="token operator">|</span> group_concat<span class="token punctuation">(</span>stuname<span class="token punctuation">)</span>               <span class="token operator">|</span> stusex <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">-------------------------------------+--------+</span>
<span class="token operator">|</span> 李斯文<span class="token punctuation">,</span>诸葛丽丽<span class="token punctuation">,</span>梅超风<span class="token punctuation">,</span>Tabm                           <span class="token operator">|</span> 女      <span class="token operator">|</span>
<span class="token operator">|</span> 张秋丽<span class="token punctuation">,</span>李文才<span class="token punctuation">,</span>欧阳俊雄<span class="token punctuation">,</span>争青小子<span class="token punctuation">,</span>Tom                          <span class="token operator">|</span> 男       <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">-------------------------------------+--------+</span>
<span class="token number">2</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><div class="language- extra-class"><pre class="language-text"><code>多学一招：【了解】
1、分组后的结果默认会按升序排列显示
2、也是可以使用desc实现分组后的降序
</code></pre></div><p><img src="http://static.zhequtao.com/study/mysql/day3/1536999889281.png" alt="1536999889281"></p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1536999987356.png" alt="1536999987356"></p> <p>多列分组</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> stuaddress<span class="token punctuation">,</span>stusex<span class="token punctuation">,</span><span class="token function">avg</span><span class="token punctuation">(</span>stuage<span class="token punctuation">)</span> <span class="token keyword">from</span> stu <span class="token keyword">group</span> <span class="token keyword">by</span> stuaddress<span class="token punctuation">,</span>stusex<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">------------+--------+-------------+</span>
<span class="token operator">|</span> stuaddress <span class="token operator">|</span> stusex <span class="token operator">|</span> <span class="token function">avg</span><span class="token punctuation">(</span>stuage<span class="token punctuation">)</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+--------+-------------+</span>
<span class="token operator">|</span> 上海          <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31.0000</span> <span class="token operator">|</span>
<span class="token operator">|</span> 北京           <span class="token operator">|</span> 女      <span class="token operator">|</span>     <span class="token number">22.0000</span> <span class="token operator">|</span>
<span class="token operator">|</span> 北京           <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">21.0000</span> <span class="token operator">|</span>
<span class="token operator">|</span> 天津           <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">27.0000</span> <span class="token operator">|</span>
<span class="token operator">|</span> 河北          <span class="token operator">|</span> 女      <span class="token operator">|</span>     <span class="token number">23.0000</span> <span class="token operator">|</span>
<span class="token operator">|</span> 河南           <span class="token operator">|</span> 女      <span class="token operator">|</span>     <span class="token number">23.0000</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+--------+-------------+</span>
<span class="token number">6</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><h4 id="_1-6-13-having条件"><a href="#_1-6-13-having条件" class="header-anchor">#</a> 1.6.13 having条件</h4> <div class="language- extra-class"><pre class="language-text"><code>思考：数据库中的表是一个二维表，返回的结果是一张二维表，既然能在数据库的二维表中进行查询，能否在结果集的二维表上继续进行查询？

答：可以，having条件就是在结果集上继续进行筛选。
</code></pre></div><p>例题</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">where</span> stusex<span class="token operator">=</span><span class="token string">'男'</span><span class="token punctuation">;</span>   <span class="token comment"># 从数据库中查找</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName  <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽         <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31</span> <span class="token operator">|</span>       <span class="token number">3</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">76</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25304 <span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">28</span> <span class="token operator">|</span>       <span class="token number">4</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>   <span class="token number">74</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25318 <span class="token operator">|</span> 争青小子        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">26</span> <span class="token operator">|</span>       <span class="token number">6</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span>   <span class="token number">86</span> <span class="token operator">|</span>   <span class="token number">92</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25320 <span class="token operator">|</span> Tom      <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">24</span> <span class="token operator">|</span>       <span class="token number">8</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">65</span> <span class="token operator">|</span>   <span class="token number">67</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token number">5</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">having</span> stusex<span class="token operator">=</span><span class="token string">'男'</span><span class="token punctuation">;</span>   <span class="token comment"># 从结果集中查找</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName  <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽         <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31</span> <span class="token operator">|</span>       <span class="token number">3</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">76</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25304 <span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">28</span> <span class="token operator">|</span>       <span class="token number">4</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>   <span class="token number">74</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25318 <span class="token operator">|</span> 争青小子        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">26</span> <span class="token operator">|</span>       <span class="token number">6</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span>   <span class="token number">86</span> <span class="token operator">|</span>   <span class="token number">92</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25320 <span class="token operator">|</span> Tom      <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">24</span> <span class="token operator">|</span>       <span class="token number">8</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">65</span> <span class="token operator">|</span>   <span class="token number">67</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token number">5</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><p>思考如下语句是否正确 <img src="http://static.zhequtao.com/study/mysql/day3/1537001246956.png" alt="1537001246956"></p> <p><img src="http://static.zhequtao.com/study/mysql/day3/1537001432304.png" alt="1537001432304"></p> <p>having和where的区别：</p> <p>where是对原始数据进行筛选，having是对记录集进行筛选。</p> <h4 id="_1-6-14-limit"><a href="#_1-6-14-limit" class="header-anchor">#</a> 1.6.14 limit</h4> <p>语法：limit 起始位置，显示长度</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">limit</span> <span class="token number">0</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">;</span>    <span class="token comment"># 从0的位置开始，取两条数据</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才       <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31</span> <span class="token operator">|</span>       <span class="token number">3</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">76</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token number">2</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
 
mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">limit</span> <span class="token number">2</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">;</span>    <span class="token comment"># 从2的位置开始，取两条数据</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName  <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25303 <span class="token operator">|</span> 李斯文        <span class="token operator">|</span> 女      <span class="token operator">|</span>     <span class="token number">22</span> <span class="token operator">|</span>       <span class="token number">2</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">55</span> <span class="token operator">|</span>   <span class="token number">82</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25304 <span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">28</span> <span class="token operator">|</span>       <span class="token number">4</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>   <span class="token number">74</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+</span>
</code></pre></div><p>起始位置可以省略，默认是从0开始</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span> <span class="token keyword">from</span> stu <span class="token keyword">limit</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">18</span> <span class="token operator">|</span>       <span class="token number">1</span> <span class="token operator">|</span> 北京           <span class="token operator">|</span>   <span class="token number">80</span> <span class="token operator">|</span> <span class="token boolean">NULL</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才       <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31</span> <span class="token operator">|</span>       <span class="token number">3</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">76</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+---------+--------+--------+---------+------------+------+------+</span>
<span class="token number">2</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><p>例题：找出班级总分前三名</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token operator">*</span><span class="token punctuation">,</span><span class="token punctuation">(</span>ch<span class="token operator">+</span>math<span class="token punctuation">)</span> total <span class="token keyword">from</span> stu <span class="token keyword">order</span> <span class="token keyword">by</span> total <span class="token keyword">desc</span> <span class="token keyword">limit</span> <span class="token number">0</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+-------+</span>
<span class="token operator">|</span> stuNo  <span class="token operator">|</span> stuName  <span class="token operator">|</span> stuSex <span class="token operator">|</span> stuAge <span class="token operator">|</span> stuSeat <span class="token operator">|</span> stuAddress <span class="token operator">|</span> ch   <span class="token operator">|</span> math <span class="token operator">|</span> total <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+-------+</span>
<span class="token operator">|</span> s25318 <span class="token operator">|</span> 争青小子        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">26</span> <span class="token operator">|</span>       <span class="token number">6</span> <span class="token operator">|</span> 天津           <span class="token operator">|</span>   <span class="token number">86</span> <span class="token operator">|</span>   <span class="token number">92</span> <span class="token operator">|</span>   <span class="token number">178</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25321 <span class="token operator">|</span> Tabm     <span class="token operator">|</span> 女      <span class="token operator">|</span>     <span class="token number">23</span> <span class="token operator">|</span>       <span class="token number">9</span> <span class="token operator">|</span> 河北          <span class="token operator">|</span>   <span class="token number">88</span> <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">165</span> <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才        <span class="token operator">|</span> 男       <span class="token operator">|</span>     <span class="token number">31</span> <span class="token operator">|</span>       <span class="token number">3</span> <span class="token operator">|</span> 上海          <span class="token operator">|</span>   <span class="token number">77</span> <span class="token operator">|</span>   <span class="token number">76</span> <span class="token operator">|</span>   <span class="token number">153</span> <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+--------+--------+---------+------------+------+------+-------+</span>
</code></pre></div><p>多学一招：limit在update和delete语句中也是可以使用的。</p> <h4 id="_1-6-15-查询语句中的选项"><a href="#_1-6-15-查询语句中的选项" class="header-anchor">#</a> 1.6.15 查询语句中的选项</h4> <p>查询语句中的选项有两个：</p> <p>1、     all：显示所有数据 【默认】</p> <p>2、     distinct：去除结果集中重复的数据</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> <span class="token keyword">distinct</span> stuaddress <span class="token keyword">from</span> stu<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token operator">|</span> stuaddress <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token operator">|</span> 上海          <span class="token operator">|</span>
<span class="token operator">|</span> 天津           <span class="token operator">|</span>
<span class="token operator">|</span> 河南           <span class="token operator">|</span>
<span class="token operator">|</span> 河北          <span class="token operator">|</span>
<span class="token operator">|</span> 北京           <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">------------+</span>
<span class="token number">5</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre></div><h2 id="_1-7-union（联合）"><a href="#_1-7-union（联合）" class="header-anchor">#</a> 1.7 union（联合）</h2> <p>插入测试数据</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">create</span> <span class="token keyword">table</span> GO1<span class="token punctuation">(</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> id <span class="token keyword">int</span> <span class="token keyword">primary</span> <span class="token keyword">key</span><span class="token punctuation">,</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> name <span class="token keyword">varchar</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Query OK<span class="token punctuation">,</span> <span class="token number">0</span> <span class="token keyword">rows</span> affected <span class="token punctuation">(</span><span class="token number">0.06</span> sec<span class="token punctuation">)</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">insert</span> <span class="token keyword">into</span> Go1 <span class="token keyword">values</span> <span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token string">'李白'</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token string">'张秋丽'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Query OK<span class="token punctuation">,</span> <span class="token number">2</span> <span class="token keyword">rows</span> affected <span class="token punctuation">(</span><span class="token number">0.02</span> sec<span class="token punctuation">)</span>
Records: <span class="token number">2</span>  Duplicates: <span class="token number">0</span>  <span class="token keyword">Warnings</span>: <span class="token number">0</span>
</code></pre></div><h4 id="_1-7-1-union的使用"><a href="#_1-7-1-union的使用" class="header-anchor">#</a> 1.7.1 union的使用</h4> <p>作用：将多个select语句结果集纵向联合起来</p> <div class="language-sql extra-class"><pre class="language-sql"><code>语法：<span class="token keyword">select</span> 语句 <span class="token keyword">union</span> <span class="token punctuation">[</span>选项<span class="token punctuation">]</span> <span class="token keyword">select</span> 语句 <span class="token keyword">union</span> <span class="token punctuation">[</span>选项<span class="token punctuation">]</span> <span class="token keyword">select</span> 语句
</code></pre></div><div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> stuno<span class="token punctuation">,</span>stuname <span class="token keyword">from</span> stu <span class="token keyword">union</span> <span class="token keyword">select</span> id<span class="token punctuation">,</span>name <span class="token keyword">from</span> Go1<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">--------+----------+</span>
<span class="token operator">|</span> stuno  <span class="token operator">|</span> stuname  <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+</span>
<span class="token operator">|</span> s25301 <span class="token operator">|</span> 张秋丽         <span class="token operator">|</span>
<span class="token operator">|</span> s25302 <span class="token operator">|</span> 李文才        <span class="token operator">|</span>
<span class="token operator">|</span> s25303 <span class="token operator">|</span> 李斯文        <span class="token operator">|</span>
<span class="token operator">|</span> s25304 <span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span>
<span class="token operator">|</span> s25305 <span class="token operator">|</span> 诸葛丽丽         <span class="token operator">|</span>
<span class="token operator">|</span> s25318 <span class="token operator">|</span> 争青小子        <span class="token operator">|</span>
<span class="token operator">|</span> s25319 <span class="token operator">|</span> 梅超风        <span class="token operator">|</span>
<span class="token operator">|</span> s25320 <span class="token operator">|</span> Tom      <span class="token operator">|</span>
<span class="token operator">|</span> s25321 <span class="token operator">|</span> Tabm     <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">1</span>      <span class="token operator">|</span> 李白         <span class="token operator">|</span>
<span class="token operator">|</span> <span class="token number">2</span>      <span class="token operator">|</span> 张秋丽         <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">--------+----------+</span>
</code></pre></div><p>例题：查询上海的男生和北京的女生</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> stuname<span class="token punctuation">,</span>stuaddress<span class="token punctuation">,</span>stusex <span class="token keyword">from</span> stu <span class="token keyword">where</span> <span class="token punctuation">(</span>stuaddress<span class="token operator">=</span><span class="token string">'上海'</span> <span class="token operator">and</span> stusex<span class="token operator">=</span><span class="token string">'男'</span><span class="token punctuation">)</span> <span class="token operator">or</span> <span class="token punctuation">(</span>stuaddress<span class="token operator">=</span><span class="token string">'北京'</span> <span class="token operator">and</span> stusex<span class="token operator">=</span><span class="token string">'女'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">---------+------------+--------+</span>
<span class="token operator">|</span> stuname <span class="token operator">|</span> stuaddress <span class="token operator">|</span> stusex <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+------------+--------+</span>
<span class="token operator">|</span> 张秋丽        <span class="token operator">|</span> 上海          <span class="token operator">|</span> 男       <span class="token operator">|</span>
<span class="token operator">|</span> 梅超风       <span class="token operator">|</span> 北京           <span class="token operator">|</span> 女      <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+------------+--------+</span>
<span class="token number">2</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>

mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> stuname<span class="token punctuation">,</span>stuaddress<span class="token punctuation">,</span>stusex <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuaddress<span class="token operator">=</span><span class="token string">'上海'</span> <span class="token operator">and</span> stusex<span class="token operator">=</span><span class="token string">'男'</span> <span class="token keyword">union</span> <span class="token keyword">select</span> stuname<span class="token punctuation">,</span>stuaddress<span class="token punctuation">,</span>stusex <span class="token keyword">from</span> stu <span class="token keyword">where</span> stuaddress<span class="token operator">=</span><span class="token string">'北京'</span> <span class="token operator">and</span> stusex<span class="token operator">=</span><span class="token string">'女'</span><span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">---------+------------+--------+</span>
<span class="token operator">|</span> stuname <span class="token operator">|</span> stuaddress <span class="token operator">|</span> stusex <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+------------+--------+</span>
<span class="token operator">|</span> 张秋丽        <span class="token operator">|</span> 上海          <span class="token operator">|</span> 男       <span class="token operator">|</span>
<span class="token operator">|</span> 梅超风       <span class="token operator">|</span> 北京           <span class="token operator">|</span> 女      <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">---------+------------+--------+</span>
<span class="token number">2</span> <span class="token keyword">rows</span> <span class="token operator">in</span> <span class="token keyword">set</span> <span class="token punctuation">(</span><span class="token number">0.02</span> sec<span class="token punctuation">)</span>
</code></pre></div><h4 id="_1-7-2-union的选项"><a href="#_1-7-2-union的选项" class="header-anchor">#</a> 1.7.2   union的选项</h4> <p>union的选项有两个</p> <p>1、     all：显示所有数据</p> <p>2、     distinct：去除重复的数据【默认】</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> name <span class="token keyword">from</span> go1 <span class="token keyword">union</span> <span class="token keyword">select</span> stuname <span class="token keyword">from</span> stu<span class="token punctuation">;</span>
<span class="token operator">+</span><span class="token comment">----------+</span>
<span class="token operator">|</span> name     <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">----------+</span>
<span class="token operator">|</span> 李白         <span class="token operator">|</span>
<span class="token operator">|</span> 张秋丽         <span class="token operator">|</span>
<span class="token operator">|</span> 李文才        <span class="token operator">|</span>
<span class="token operator">|</span> 李斯文        <span class="token operator">|</span>
<span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span>
<span class="token operator">|</span> 诸葛丽丽         <span class="token operator">|</span>
<span class="token operator">|</span> 争青小子        <span class="token operator">|</span>
<span class="token operator">|</span> 梅超风        <span class="token operator">|</span>
<span class="token operator">|</span> Tom      <span class="token operator">|</span>
<span class="token operator">|</span> Tabm     <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">----------+</span>
</code></pre></div><p>默认是去重复的</p> <div class="language-sql extra-class"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">select</span> name <span class="token keyword">from</span> go1 <span class="token keyword">union</span> <span class="token keyword">all</span> <span class="token keyword">select</span> stuname <span class="token keyword">from</span> stu<span class="token punctuation">;</span>  <span class="token comment"># all不去重复记录</span>
<span class="token operator">+</span><span class="token comment">----------+</span>
<span class="token operator">|</span> name     <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">----------+</span>
<span class="token operator">|</span> 李白         <span class="token operator">|</span>
<span class="token operator">|</span> 张秋丽         <span class="token operator">|</span>
<span class="token operator">|</span> 张秋丽         <span class="token operator">|</span>
<span class="token operator">|</span> 李文才        <span class="token operator">|</span>
<span class="token operator">|</span> 李斯文        <span class="token operator">|</span>
<span class="token operator">|</span> 欧阳俊雄        <span class="token operator">|</span>
<span class="token operator">|</span> 诸葛丽丽         <span class="token operator">|</span>
<span class="token operator">|</span> 争青小子        <span class="token operator">|</span>
<span class="token operator">|</span> 梅超风        <span class="token operator">|</span>
<span class="token operator">|</span> Tom      <span class="token operator">|</span>
<span class="token operator">|</span> Tabm     <span class="token operator">|</span>
<span class="token operator">+</span><span class="token comment">----------+</span>
</code></pre></div><h4 id="_1-7-3-union的注意事项"><a href="#_1-7-3-union的注意事项" class="header-anchor">#</a> 1.7.3   union的注意事项</h4> <p>1、     union两边的select语句的字段个数必须一致</p> <p>2、     union两边的select语句的字段名可以不一致，最终按第一个select语句的字段名。</p> <p>3、     union两边的select语句中的数据类型可以不一致。</p></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">11/18/2020, 2:52:26 PM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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